Large deposits of marine phosphorites exist in the United States, particularly in Florida and the Western United States. These deposits are known to contain small amounts of uranium (e.g., about 50 to 200 ppm or 0.1 to 0.4 pound per ton of phosphate rock). Such ores are a valuable source of uranium.
Processes are known for producing phosphoric acid from phosphate rock by acidulating the rock with sulfuric acid, such processes being known as the "wet process". In this process, the phosphate rock is converted to a relatively dilute phosphoric acid solution, calcium sulfate also being formed in the reaction. The calcium sulfate is filtered out to provide an impure acid stream which typically contains about 30% by weight of P.sub.2 O.sub.5 and the uranium originally present in the ore. Processes have been proposed for recovering uranium from the phosphoric acid solution using solvent extraction techniques selective to uranium.
However, the impure 30% acid, which can be either "black" or "green". contains measurable amounts of organic material, such as humic matter, which can have an adverse effect on the solvent extraction efficiency of uranium.
As with any solvent extraction process, it is important that the feed to the solvent extraction section be free of any particulate matter in order to avoid the formation of stable emulsions commonly called crud.
In cleaning impure phosphoric acid, filtration generally removes the suspended particulates but not the colloidal matter composed of organic or "humic" material.
The organic or humic matter referred to hereinabove is sometimes commonly referred to as "humic acid" or "humates". This material is not a well defined compound but is a mixture of polymers containing aromatic and heterocyclic structures, carboxyl groups and nitrogen. By colloidal humic matter is meant humic matter which has a particle size in the range of from about 1 to about 500 millimicrons.
When black phosphoric acid which is cleaned by filtration is subsequently processed by solvent extraction in order to recover uranium, there results an agglomeration and precipitation of the colloidal humic content of the acid and an intolerable build-up of solids or crud at the organic/aqueous interface of the solvent extraction circuit.
Several processes have been proposed for removing organic material from wet-process phosphoric acid.
In U.S. Pat. No. 4,087,512, a process is disclosed for removing solid organic material from wet-process phosphoric acid produced by the acidulation of uncalcined phosphate rock. The process utilizes an insoluble hydrocarbon liquid which is mixed with the phosphoric acid solution to effect selective removal of the organic matter. In summary, the process comprises mixing the hydrocarbon (e.g., kerosene) at a temperature not less than about 55.degree. C. and not more than about 70.degree. C. with the wet-process solution at a volume ratio of aqueous to organic of up to about 30:1, allowing the mixture to separate into an aqueous phase and an organic phase containing the solid organic matter and then separating the cleaned phosphoric acid from the hydrocarbon phase.
In a preferred embodiment, the volume ratio of aqueous to the organic liquid phase ranges from about 1:1 to 2:1. A disadvantage of this process is the large amount of hydrocarbon required for carrying out the process.
In U.S. Pat. No. 4,064,220, a process is proposed for purifying acid solutions of dissolved or colloidally dispersed organic matter by mixing with an acid solution an aldehyde or an aldehyde oligomer and a compound copolymerizable with the aldehyde (e.g., phenol) and allowing polymerization to take place. The polymer that forms separates from the solution and removes with it the organic matter. Alternatively, the aldehyde and the copolymerizable compound can be added as a pre-condensate to obtain the same result.
It would be desirable to provide a process which does not require the use of substantial amounts of reagents for removing humic matter from impure phosphoric acid solution. As a result of a search for such a process, a method has now been developed which enables the removal of humic matter from phosphoric acid in a simple and direct manner using a small but effective amount of an addition agent.